MigrateInstance(target) before Object.assign(target, ...)

test/mjsunit/regress/regress-object-assign-deprecated.js

Index: test/mjsunit/regress/regress-object-assign-deprecated.js
diff --git a/test/message/regress/regress-4829-1.js b/test/mjsunit/regress/regress-object-assign-deprecated.js
similarity index 72%
copy from test/message/regress/regress-4829-1.js
copy to test/mjsunit/regress/regress-object-assign-deprecated.js
index 1ad5feda8cb96cd24e4c47f7407c9699d150c327..d2e60f99e51c7d12c37d98e7d3b320af2e8e752c 100644
--- a/test/message/regress/regress-4829-1.js
+++ b/test/mjsunit/regress/regress-object-assign-deprecated.js
@@ -2,6 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-function tag() {}
-
-tag(tag`\xyy`);
+var x = {a:1, b:2};
+var y = {a:1, b:2.5};
+Object.assign(x, x);